The present invention generally relates to underground storage tanks for storing gasoline dispensed at filling stations and, more particularly, to an underground storage tank unit having a drop tube sealing system for preventing the escape of underground storage tank ullage vapors.
During the filling of underground storage tanks (UST""s) for fuel, as shown in FIG. 1, a tanker truck generally delivers fuel to a fill riser 26 through a fill adaptor 20 and direct fill spill containment bucket 24. The spill containment bucket is connected to a riser pipe 28 that extends from the top of the underground tank 30. Fuel is delivered from the fuel tanker to the fill riser via a fuel duct that attaches to top of the fill riser. A tube 32 (herein referred to as a xe2x80x9cdrop tubexe2x80x9d) that extends from the top of the fill riser to location below the stored fuel level 40 is used to prevent fuel entering the UST from splashing and agitating liquid fuel stored within the tank. The drop tube is concentrically disposed within the fill riser and riser pipe such that an annular channel 37 is formed between them. The annular channel routes any excess fuel that is spilled from the top of the fill riser into the containment bucket to the UST via a drain channel 39 coupled between the bucket floor and the riser pipe. A poppet drain valve 35 disposed in the containment bucket allows the excess fuel to pass through the drain channel. An O-ring seal 36 is provided between the top of the fill riser and the fill adaptor to prevent fuel vapor from rising along the annular channel to the top of the fill riser and escaping into the atmosphere.
Filing stations typically utilize a vapor recovery system to capture fuel vapors being displaced from a customer""s tank by the fuel being added to the vehicle tank. Such systems provide a partial vacuum to draw excess fuel vapor from the customer""s tank and route them back to the station""s underground tanks. The captured fuel vapors cause fuel vapor pressure changes in the UST ullage space. As a result, the UST may become pressurized by the cumulative effects of the vapors produced therein, and the sloshing and agitation of the stored fuel during filling of the tank.
The higher pressure in the UST ullage space often causes emission of vapors through the annular channel 37 which may leak through defective seals and fittings associated with the spill containment bucket assembly. Additionally, the fumes and vapor pressure that rise from the UST within the annular passage may prevent drainage of excess fluid from the containment bucket, particularly when the fuel is warmed, such as on hot summer days. The drain valve at the containment box is effectively inoperative when excess pressure is exerted on the valve from below. Further, since the spill containment bucket may be exposed to the surface, such as when a manhole cover 22 sealing the containment bucket is removed, often dirt and other foreign matter degrade the seal in the drain valve unit. This problem is exacerbated in sites that use remote UST fills since the entire remote fill piping and spill bucket are exposed to the UST ullage vapors.
Attempts have been made to solve a portion of the problem. In one example, the seal between the drop tube and the riser pipe is re-located from atop the intake tube to a location along the riser pipe, just below the containment bucket. While this system has been helpful in preventing vapors from emitting to the atmosphere through the direct fill containment bucket, fuel vapors are still capable of escaping to the atmosphere through piping connected to remote fill spill bucket assemblies.
Thus, there is a need for a drop tube sealing assembly which helps prevent any of the UST ullage vapors from escaping out of either the direct or remote fill spill bucket assemblies. Such an assembly should be adapted to be easily installed in existing USTs.
The present invention is generally directed to drop tube sealing assembly for sealing direct and remote spill containment buckets, and associated assemblies, from an underground tank ullage vapor pressure. In one embodiment, the drop tube sealing assembly includes a riser tube having a proximal end with internal threads and an underground storage tank spaced apart from the proximate end of the riser tube. The storage tank contains a threaded inlet which is positioned atop the tank. A standard pipe nipple includes a first end in cooperation with the proximal end, an opposite end in cooperation with the threaded inlet, and an annular inner surface that forms a conduit. The inner surface comprises a female thread section. A drop tube adapter fitting is concentrically disposed within the pipe nipple. The adapter fitting contains an outer surface containing external threads in cooperation with the female thread section. A seal in the form of an O-ring is disposed between the adapter fitting and the pipe nipple. A drop tube having a open end is coupled to the adapter fitting.
The present invention provides an improved method and device for containing underground tank ullage vapor pressure that generally enter the direct and remote spill containment buckets, and associated assemblies.